Explosive-detonation and autoignition of end gas in a combustion chamber produce a metallic sound, commonly referred to as knock. Knock is caused by improper ignition of fuel in an internal combustion engine. Improper ignition results in decreased engine performance and increased emissions. Knock, furthermore, generates acoustic vibrations which propagate throughout the engine structure, and possibly other adjoining structures. These vibrations, coupled with a rapid rate of pressure rise in the combustion chamber, may promote accelerated wearing of engine components. Wear may be even faster for engines operating with natural gas, due to a higher rate of pressure rise in the combustion chamber as compared with gasoline powered engines.
Prior systems provide means for detecting knock and controlling selected engine operating parameters to reduce the knock to an acceptable level. Recently these efforts have been directed to sensing knock induced vibrations by monitoring one or more characteristic frequencies corresponding to the acoustic cavity resonance modes of the combustion chamber. These characteristic frequencies generally act as carrier waves and modulate the knock vibrations. When demodulated, the magnitude of the envelope of the carrier wave denotes the magnitude of the knock. This information is typically used to retard the spark advance, which reduces knock.
Air/fuel ratio controls and spark retarders effectively reduce knock to acceptable levels. However, these controls and their sensors can malfunction. Known oxygen sensors, in particular, have relatively short lives of 1000 to 2000 hours. In automotive applications this life expectancy is acceptable. However, work engines may be required to perform in excess of 10,000 hours. Should a sensor or control fail, the engine would be susceptible to possibly damaging knock.
The present invention is directed to overcoming one or more of the problems as set forth above.